Golden Silence

This appeared in the December 1993 issue of the Falco Builders Letter.

by Stephan Wilkinson

There are two things you can do to your airplane-one cheap, the other expensive-that will do more to make it smooth and quiet than all the soundproofing material it might be able to lift. The first is balancing the prop (the cheap one), and the second is buying a Bose active-noise-reduction headset (at $1,000 apiece, expensive).

Having a propeller tuned using a Chadwick-Helmuth electronic balancer costs about $150, and the difference it made on my Falco was perceptible on the first post-balancing flight. The engine ran with nearly the same smoothness of a BMW six, even though it's a 180-hp Lycoming with four ashtray-size pistons each displacing 1,500cc-the equivalent of four Honda Civic engines in very close formation. And this was an airplane that already was in a good state of balance, according to the technician who did the job.

The balancing process is simple. It requires uncowling at least the top half of the engine, and bolting a small transducer to the crankcase centerline at the front. When the engine is run at 2,000 rpm, that transducer senses the vigor, in Gs, with which the entire engine is moving longitudinally off its centerline. A strobe-light gun fired at the prop disk determines the location of the out-of-balance node.

A powerplant in perfect balance will give a reading of zero (equivalent to one G). a reading of .2 is considered "acceptable." The Falco came in at .3 before balancing, and many typical lightplanes are enough out of balance that readings as high as .8 aren't unusual. A powerplant that reads 1.2 or higher is so out of balance that simply adding counterweights to the prop spinner or the starter ring-gear flywheel won't solve the problem. Either the prop must be removed and balanced independently or there's an internal engine problem.

My mechanic was able to get the Falco down to a reading of .05-nearly perfect-with a single lucky guess as to exactly where 26 grams of balance weight should be bolted to the flywheel. "This is the easiest job I've ever done," he laughed. "You can sometimes spend three hours trying to find the sweet spot."

I flew the airplane immediately, and even the minute buzz that had always been perceptible through the stick had disappeared. Prop balancers say the biggest benefits of the process are the elimination of cowling cracks, wire breakages, landing light filament failures, accessory-mounting cracks, exhaust manifold splits, leaky oil coolers and the like, and I believe them. Ancillary benefits are stroboscopic verification of engine speed, so you can check your tachometer accuracy, and early detection of certain internal engine faults. At $150 for all this, prop balancing may be general aviation's last true bargain.

It's hard to call Bose headsets a bargain, since it'll take about $4,000 to fully equip a four-seater. Other companies are marketing active-noise-reduction headsets for discounted prices of as little as $340 to $850 apiece. (Bose sells only factory-direct, no discounts.) Since Bose is basically a stereo electronics-and-speakers manufacturer, the suspicion lingers that there is less than the usual relationship between price and quality, for the hi-fi industry is one in which product "improvements" sometimes are hyped by extreme prices rather than by perceptible improvements.

But I have one friend who bought a cheaper ANR unit and finds the improvement in sound quality dubious enough that he no longer bothers with the fuss of plugging the power unit into the cigarette lighter; he simply uses them as conventional noise-attenuating earphones. And another friend, who flies a Mooney, owns an avionics shop and sells Bose's competitors, admitted when he saw my new Bose headset, "Yeah, they're the ones to buy, if you can afford them. They're the best." Strong-if informal-recommendations.

If you'd like to be the first on the block to have one, you might try contacting the factory at Bose Corporation, The Mountain, Framingham, Massachusetts 01701. The fax number of 508 872-8928 or you can call 800-637-8781, an infuriating toll-free voicemail recorded exchange.

Unless you fly a Mooney, do not order the coiled-cord model; it's too short and tightly coiled to work at conventional headset-to-panel distances. Your other choice is a free-standing or built-in electronic-interface unit. The former plugs into a cigarette lighter and can be carried from airplane to airplane. The latter can be mounted on or under the instrument panel, or on a cockpit sidewall, and hardwired to the airplane's audio system. It's much neater, but limits your Bose headset to use only in that airplane.

I'd initially tried a Bose headset in a Cessna 205, and the improvement in quietude was incredible. There's no mistaking the difference, either, for the ANR portion of the Bose is powered by the interface unit and can be switched on and off. With the power off, the Bose is an excellent conventional noise-attenuating headset. Turn the power on, however, and low-frequency noise is immediately canceled out, swallowed up, sucked away, magically eliminated.

Interestingly, in the Falco the effect is not quite so extreme, even though the Falco's cockpit is a louder environment than is any Cessna's. Most of the engine, exhaust and propeller noise-all low-frequency-are eliminated by the Bose, but what's left is the higher-frequency noise of fast-moving airflow over the big canopy, something that apparently is not much of a factor in a plodding old Cessna 205. The result is a moderate sibilance, a hiss, very much like the sound you'd experience in a Learjet cockpit. I'm not complaining; it makes my little Italian stallion feel even faster.